Refactor Models into Per-Package Package

[da1910]

Currently models are all in the same ansys.materials.manager._models namespace. This is private, and contains models for all the supported solvers.

This is inconvenient, as it doesn't make the distinction between common and per-package models clear, and it means that users need to import a private module.

We should look at improving this, but there are a few things to think about...

1. Do we want to separate these modules/packages out into separate packages?
2. Do we want to import them into the main library, or do we leave them as external imports?
3. How do we handle the case where more than one, but not all the package support the same model? This might make common substantially bigger than any of the solver specific packages.